1. Field of the Invention
The present invention relates to a polarization independent optical isolator using a Faraday rotator made of a magnetic garnet single crystal, and two wedge-shaped birefringent crystal plates, and more particularly to improvement of a polarization independent optical isolator, which prevents deterioration in characteristics and breakage of a magnetic garnet single crystal associated with rise in temperature due to light absorption by the magnetic garnet single crystal.
2. Description of the Related Art
An optical isolator is a non-reciprocal optical device having a function of allowing an optical signal to pass therethrough in the forward direction and preventing an optical signal from passing therethrough in the reverse direction. For example, an optical isolator is used in an optical communication system using a semiconductor laser as a light source in order to prevent oscillation of the semiconductor laser from being unstable due to returning of a reflected optical signal to the light source side.
Optical isolators can be broadly classified into polarization dependent optical isolators such as those used for semiconductor laser modules, and polarization independent optical isolators such as those used in front of and behind optical fiber amplifiers.
In a general polarization independent optical isolator, two wedge-shaped birefringent crystal plates made of rutile, YVO4, LiNbO3, or the like are used as polarizers, and a flat plate made of a magnetic garnet single crystal is placed, as a Faraday rotator, between the two wedge-shaped birefringent crystal plates (see Patent Document 1: U.S. Pat. No. 4,548,478, and Patent Document 2: U.S. Pat. No. 5,315,431). Here, the Faraday rotator has a crystal thickness adjusted so that the Faraday rotator can rotate the polarization by 45 degrees, and the two wedge-shaped birefringent crystal plates are placed such that the directions of the optic axes thereof are shifted from each other by 45 degrees. Note that an optical element unit including the polarizers and the Faraday rotator is referred to as a non-reciprocal unit.
Light incident on a first wedge-shaped birefringent crystal plate in the forward direction is separated into an ordinary ray and an extraordinary ray by the first wedge-shaped birefringent crystal plate. Since the Faraday rotator rotates the polarization by 45 degrees, and since the optic axis of a second wedge-shaped birefringent crystal plate is shifted from that of the first wedge-shaped birefringent crystal plate by 45 degrees, the ordinary ray is incident on the second wedge-shaped birefringent crystal plate as an ordinary ray, and the extraordinary ray is incident thereon as an extraordinary ray. Then, the rays exit as parallel light from the second wedge-shaped birefringent crystal plate, and are coupled with an optical fiber by a collimator lens.
Light traveling in the reverse direction is separated into an ordinary ray and an extraordinary ray by the second wedge-shaped birefringent crystal plate. After rotated by the Faraday rotator by 45 degrees, the ordinary ray is incident on the first wedge-shaped birefringent crystal plate as an extraordinary ray, and the extraordinary ray is incident thereon as an ordinary ray. Hence, light exiting from the first wedge-shaped birefringent crystal plate is not parallel, and is not coupled with the core of the optical fiber. The optical isolator functions in such a manner.
Here, the magnetic garnet single crystal constituting the Faraday rotator hardly involves problems in the near-infrared wavelength region, in particular, around a wavelength region (1.2 μm to 1.7 μm) used for optical communications. This is because, in such a region, the magnetic garnet single crystal exhibits an excellent optical transparency, and only slightly undergoes temperature rise due to absorption of light, as long as the light is about several hundreds milliwatts. However, in an shorter wavelength region than the above-described wavelength region, in particular, in the wavelength region around 1 μm, which is employed for excitation light of YAG lasers, fiber lasers which attract an attention as alternatives to the YAG lasers, and optical fiber amplifiers, the absorption of light by the magnetic garnet single crystal is increased, resulting in a non-negligible temperature rise at an laser power of several hundreds milliwatts.
A paramagnetic single crystal or a paramagnetic glass may be used as a Faraday rotator used for an optical isolator designed for the wavelength region around 1 μm. However, when such a material is used, not only the size of the Faraday rotator itself is increased, but also a large magnet is required to magnetically saturate the Faraday rotator. As a result, the optical isolator also becomes large.
In this respect, an optical isolator for the 1-μm band has been required to be small, and to withstand laser light with a high-power, recently.
Patent Document 3 (Japanese Patent Application Publication No. 2007-256616) describes a polarization independent optical isolator including c-plane sapphire single crystal plates bonded to optical surfaces of a magnetic garnet single crystal to suppress the temperature rise. This configuration is intended to achieve an optical isolator having high-power-laser resistance, while a magnetic garnet single crystal is used as a Faraday rotator.
However, in the case of the polarization independent optical isolator described in Patent Document 3, when the magnetic garnet single crystal (Faraday rotator) to which the c-plane sapphire single crystal plates are bonded is incorporated into an optical isolator, the magnetic garnet single crystal (Faraday rotator) needs to be individually placed while being inclined, such that the angle of light incident on each of the c-plane sapphire single crystal plates is 1 to 6 degrees with respect to the c-axis. This leads to a disadvantage that the assembly costs of the optical isolator is increased.
In this respect, Patent Document 4 (Japanese Patent Application Publication No. 2010-048872) proposes a polarization independent optical isolator with which the disadvantage of Patent Document 3 is eliminated.
Specifically, the polarization independent optical isolator is characterized in that a light transmitting surface of each of the sapphire single crystal plates is formed in such a manner as to be parallel to a non-inclined light transmitting surface of a corresponding adjacent one of the wedge-shaped birefringent crystal plates, and to be offset from a c-plane of the sapphire single crystal plate, and the bisector of an angle formed by optical axes of an ordinary ray and an extraordinary ray separated by each of the wedge-shaped birefringent crystal plates is perpendicular to the c-plane of each of the sapphire single crystal plates (in other words, the c-plane of a sapphire single crystal plate 2 is offset by an angle θ formed by the bisector αof the angle formed by the optical axes of an ordinary ray and an extraordinary ray separated by the wedge-shaped birefringent crystal plate and an axis perpendicular to a light transmitting surface of the sapphire single crystal plate 2, as shown in FIG. 7).
Note that the term “offset” is a term mainly used in the field of crystal growth and the like. For example, a sapphire single crystal plate obtained by cutting a sapphire ingot along a plane perpendicular to the c-axis is referred to as a “c-plane sapphire single crystal plate,” whereas a sapphire single crystal plate obtained by cutting a sapphire ingot along a plane inclined from a plane perpendicular to the c-axis by an angle θ is referred to as a “sapphire single crystal plate having an offset angle θ from the c-plane.”
The polarization independent optical isolator described in Patent Document 4 is characterized in that the bisector of an angle formed by the optical axes of an ordinary ray and an extraordinary ray separated by the wedge-shaped birefringent crystal plate is perpendicular to the c-plane of each of the sapphire single crystal plates (in other words, the c-plane of each of the sapphire single crystal plates is offset by an angle formed by the bisector and the axis perpendicular to the light transmitting surface of the sapphire single crystal plate). Here, when an effect of refraction of imaginary light represented by the bisector incident on the sapphire single crystal plate is taken into consideration, the peak isolation may be about 35 dB in some cases. There has still been a problem that it is difficult to fabricate an optical isolator having such a high performance that the peak isolation can be 40 dB or more stably. This is because, in the case where the refraction of the imaginary light represented by the bisector in the sapphire is taken into consideration, it is impossible to precisely express the angle formed by the beam of light in the sapphire and the c-axis of the sapphire by use of the bisector of the angle formed by the optical axes of the ordinary ray and the extraordinary ray separated by the wedge-shaped birefringent crystal plate. As a result, the angle formed by the ordinary ray and the c-axis, or the angle formed by the extraordinary ray and the c-axis may be larger than the angle formed by the bisector and the c-axis in some cases, which may lead to deterioration in extinction ratio. For this reason, the peak isolation may be less than 40 dB depending on the polarization in some cases.
Moreover, when the polarization independent optical isolator described in Patent Document 4 is used as an optical isolator for a fiber laser, the following problem arises. Specifically, when the optical feedback couples with the cladding of an incident side optical fiber, and propagates through the cladding, an optical system placed on the inlet side of the incident side optical fiber may be damaged, because the intensity of the optical feedback passing through the optical isolator is higher than that of an optical isolator for optical communication.